Revenue and Customs is planning to raid fashion companies employing unpaid interns in breach of minimum wage laws, the Guardian has learned.

Internal documents from Her Majesty's Revenue and Customs show it believes interns across the employment spectrum to be at "high risk" of abuse under national minimum wage laws, and that HMRC has convened a 12-person taskforce to make unannounced inspections of businesses where interns are being used as workers rather than just shadowing staff.

The special "dynamic response" unit will have powers to question managers and sift through accounts until it is satisfied that no abuse is taking place.

It is the first time intern abuse has been targeted by the HMRC, which is responsible for the enforcement of the minimum wage. It follows heavy criticism from the Low Pay Commission over the lack of enforcement action.

Companies such as clothes store Urban Outfitters have advertised for nine-month unpaid internships.

Topshop, owned by billionaire Sir Philip Green, also offers month-long unpaid internships although it says the role only involve shadowing employees, not actual work.

Fashion house Vivienne Westwood has previously advertised for a three-month unpaid internship which required a high level of prior IT skills from would-be applicants.

The HMRC internal briefing document also highlights the growing nature of the intern problem.

It says that over the past few years there has been a significant rise in the number of UK employers offering internships. It adds that the HMRC should target the fashion industry for the next six months because it is "well known for the use of interns".

An HMRC source told the Guardian it would be targeting a range of firms, from the multinationals to the smallest design company.

"If you are in that industry and you have interns, there's no reason to think that we won't be knocking on your door," they said.

The source also said the campaign was aware of the sensitivities about approaching interns themselves who may fear being "blacklisted" in later employment.

"We do appreciate that we might not want to walk up to an intern and ask, 'Are they paying you?'

"Obviously by the time we leave, life might not be so great for the intern at that place. We are aware that there is a sensitivity there."

The source added that the team would have the power to "look at whatever they need to look at to assure themselves that the people who are working there are getting paid what they are due".

Rachel Johnson, editor of lifestyle and fashion magazine The Lady, said her interns only come for a week and they are offered expenses and lunch money, and that anyone who is asked to stay on for longer is paid a wage.

She said: "We don't have any long-term unpaid interns … because it is not fair on them. It is inequitable only to take on the children or young people of families who can afford to subsidise them.

"It's unfair to those who cannot afford to work and not be paid … I think the whole new model economy of unpaid work is not a good one for entry-level employment at all."

Tanya de Grunwald, founder of website Graduate Fog and who is running a name and shame campaign called Pay Your Interns, said: "I am pleased to see HMRC at last taking the illegal internships issue seriously.

"For over a year now, I and the other interns' rights campaigners have been complaining loudly that the existing reporting system – which requires exploited interns to blow the whistle on their own employer – was fatally flawed.

"Unpaid Interns are desperate for experience and a reference so why would they report somebody they are trying to impress?

"We told HMRC that the burden must be removed from the interns' shoulders. It seems that they have listened at last."

She added: "HMRC is right in thinking that fashion is one of the worst industries for taking advantage of their young workers.

"The thought of being spot-checked will have the fashion houses' lawyers shaking in their expensive shoes."